The dhroud of Turin Results of recent I4Cdating of the Shroud of Turin using tandem accelerator mass spectrometry indicate that the cloth was created between 1260 and 1390 A.D.
FOCUS he Shroud of Turin is a piece of herringbone weave linen about 14 ft long and 3 ft wide that hears the faint image of a crucified bearded man. It is kept in a silver casket in Turin’s Cathedral of St. John the Baptist and displayed only a few times each century. The Shroud first surfaced during the 1350s in Liray, France, when a French knight named Geoffrey de Charny displayed the cloth at the opening of a new church, claiming that the Shroud was the long sought-after burial cloth of Christ. Almost immediately the cries of forgery began. The Bishop of Troyes declared that the Shroud was a fake that had been painted by a local artist. The controversy continued through the next six centuries until 1978,when an international team of scientists known as the Shroud of Turin Research Project, or STURP, received permission from the Roman Catholic Church to study the Shroud. Twenty-four hours a day for five days, they probed the Shroud with every nondestructive technique they could think of, including X-ray fluorescence and ultraviolet and infrared spectrometry. They also received permission to use sticky tape to remove a few fibers from the Shroud for microscopic analysis. In 1981 the STURP finally announced its results. The group, with one notable exception, concluded that the Shroud was not a forgery and that the red stains around the man’s hands, feet, and side were blood stains testing positive for hemoglobin and serum albumin. One member of the group, however, concluded that the Shroud was a four-
T
teenth-century forgery. Walter McCrone of McCrone Associates in Chicago, a microscopist specializing in the authentication of art objects, believed that the Shroud contained a pale, gelatin-based medium specked with particles of red ocher, which is made from a natural iron oxide and has been used as a pigment for thousands of years. He also found particles of a synthetic form of vermilion that was developed in the Middle Ages.
McCrone thus concluded that the Bishop of Troyes had been right and that the Shroud was painted by a fourteenth-century artist. Radiocarbondating Although the STURP concluded that the Shroud W a s not a forgery, the controversy continued. The only apparent way to end the speculation about the Shroud‘s authenticity was to subject it to radiocarbon dating. Traditional
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Flgure 1. Schematic of the tandem accelerator mass spectrometer at the University of Arizona. means of such dating, however, would have required that a piece of the Shroud as large as a handkerchief be sacrificed, and Cardinal Anastasio Ballestrero, Archbishop of Turin and Pontifical Custodian of the Shroud, was understandably leery of such a prospect. As radiocarbon dating technology advanced, and the emergence of tandem accelerator mass spectrometry (TAMS) dating methodology decreased the size of sample required for accurate dating, however, scientists again began to clamor for a chance to determine the age of the Shroud. In 1983 Umberto I1 of Savoy, the exiled king of Italy and owner of the Shroud, willed it to Pope John Paul I1 and his successors, clearing the way for the Church to respond to the increasing pressure for more scientific testing of the Shroud. Finally, on October 1, 1986, the Archbishop of Turin and the Pontifical Academy of Sciences jointly sponsored a meeting on radiocarbon dating of the Shroud. Participants in the workshop recommended a protocol involving both TAMS and the more traditional proportional counting technique by seven laboratories in Europe and the United States. But even the design of the radiocarbon dating studies caused considerable controversy. In October 1987 the Archbishop of Turin notified the seven laboratories preparing to undertake the radiocarbon dating that only threethe University of Arizona, Oxford University in England, and the Technical University of Zurich-would be allowed to study the Shroud. The Archbishop contended that removing seven 102A
pieces from the Shroud, even seven small ones, would be too destructive. In addition, because TAMS requires a sample only one-third the size of that required for proportional counting, the Archbishop decided to limit the testing to laboratories experienced in dating archeological samples using TAMS. The protests from proportional counting advocates as well as from the rejected laboratories came fast and furious, with claims that accelerator technology was not yet ready to do what the Church wanted it to do, primarily because of the frequency of spurious readings from small samples. Nevertheless, the Archbishop stuck to his plan to allow only three postagestamp-sized samples to be taken from the Shroud. In January 1988 directors of the three selected laboratories met with Luigi Gonella, scientific advisor to the Archbishop of Turin, and Michael Tite, head of research a t the British Museum (who had been asked by the Archbishop to assist in certifying the operation), to accept the Vatican’s decision to use no more than three samples in the interests of conservation of the Shroud. The University of Arizona was represented by geoscientist PaulE. Damon and physicist Douglas J. Donahue, joint directors of the National Science Foundation-Arizona Accelerator Facility for Radioisotope Analysis. The procedures agreed upon by the three labs and the Vatican were as follows: Each laboratory would be provided with a sample from the Shroud, together with two control samples of known age, one of which had been independently dated by conventional radiocarbon dating. The samples would each weigh 40 mg and would be
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taken from a single site on the Shroud away from any patches or charred areas. Each sample, including the controls, would be weighed, wrapped in aluminum foil, and sealed in numbered stainless steel containers. Each of the three laboratories would then subject all three samples to radiocarbon dating by TAMS and submit their results to Tite at the British Museum. TAMS dating Radiocarbon dating is based on the slow decay of lac, which is produced when cosmic rays from space strike the Earth’s atmosphere. Following production, 14C is rapidly incorporated into atmospheric carbon dioxide and then becomes part of all living things, as plants take in 14C02,and the plants in turn are eaten by animals and humans. The ratios of the three carbon isotopes remain more or less constant during life, but when an organism dies, the decaying radioactive carbon is no longer replenished from the environment. The 14Calready present in the tissues continues to decay at a constant rate, however, and hy measuring the ratio of 14Cto the stable isotopes in an object, scientists can determine the age of the object. The half-life of 14C is -5700 years, and, using current technology, researchers have found that radiocarbon dating can provide accurate dates for material as old as 45,000 years. As the TAMS method is perfected, scientists predict that it may provide accurate dates for materials as old as 60,000 years. Radiocarbon dating using TAMS is a complex process involving combustion of the sample to convert the carbon in the sample to carbon dioxide, conver-
sion of the carbon dioxide to graphite, compression of the graphite into target pellets, and analysis of the graphite pellets by TAMS. A schematic of the TAMS instrument used at the University of Arizona is shown in Figure 1. TAMS is particularly sensitive compared with traditional proportional counting because this instrument can directly measure each 14C atom in an object, whereas proportional counting can only measure the radioactive decay of the 14Cin the sample. After the graphite targets (including NBS standard targets containing precisely known quantities of all three carbon isotopes) are loaded into the instrument, a beam of cesium ions is fired at the graphite target, transforming neutral carbon atoms into negatively charged ions. The 12C,13C,and 14Cions are pulled off of the graphite surface by a powerful electric field and sent toward a 90° injection magnet. Because the carbon isotopes have different masses and thus different energies, they are separated and move into the accelerator at different speeds. The negative ions are then sent into an argon “stripper,” where the carbon ions collide with argon atoms, losing four electrons. (This step removes background ions that have the same mass as the carbon isotopes.) The C3+ ions then proceed through the accelerator toward a focusing magnet that deflects the 14C ions through a foil and into a solid-state detector. (The 12C and l3C ions are measured as a beam current using Faraday cups prior to the 14Cdetector.) The ratio of the three carbon isotopes is then calculated, leading to an estimate of the age of the sample.
Testing the Shroud Researchers at the three selected labs witnessed the cutting of the Shroud last April in Turin and soon began tests to determine the age of the flax in the linen. “We had originally planned that the test be a blind test,” said Damon. “But it soon became apparent that the British Museum could not match the weave of the Shroud with appropriate control samples, and the concept of a blind test was abandoned.” Although the scientists knew which sample was the Shroud, they did not know the age of any of the samples. At the University of Arizona, scientists prepared eight carbon targets from the Shroud sample and measured two targets each on four days about a week apart. To ensure dating accuracy, each measurement was taken using twice the number of calibrating targets normally ‘used.They statistically averaged their results and forwarded them
to Tite’s laboratory at the British Museum, where the final 14C measurements submitted by each lab were averaged and converted to a calendar date. In August, Tite returned the statistical analysis and calendar date to all three labs. “The consistency between the University of Arizona results and those of the other labs was very good and gratifying,” said Donahue. Following approval by all three labs, a final report was sent by special courier to Turin, and Cardinal Ballestrero forwarded the results to Rome. Finally, on October 13, Ballestrero announced the results: The calibrated calendar age range of the Shroud at the 95% confidence level is 1260 to 1390 A.D. A detailed description of the study and the results is being prepared for publication by Tite and the representatives of the three labs.
The mystery remains Although it has now been proven that the Shroud of Turin is not, as legend has it, the burial cloth of Christ, there are still questions about how the image on the Shroud was formed. McCrone ( I ) contends that the Shroud is a painting. Most investigators, however, believe that the image was not made by any known technique of painting or staining, and that it looks just like a photographic negative-produced centuries before photography was invented. “The problems of the origin of the image and its conservation are still left mostly unsolved and will need further research and study,” said Cardinal Ballestrero in his statement announcing the results of the 14Cdating. Although the church has never officially proclaimed the Shroud to be Christ’s burial cloth, it hasn’t discouraged those who chose to believe the legend. After announcing that the Shroud originated in the Middle Ages, the Church “reaffirmed its respect and veneration for this venerable icon of Christ, that remains an object of cult for the faithful in coherence with the position always expressed with regard to the Shroud.” Until science comes up with a way to determine how the image on the Shroud was made, the Shroud of Turin remains as mysterious as ever. Mary Warner Reference (1) McCrone,W. Wiener Berichte uber Naturwrssenschaft in der Kunst, in press.
Suggested readlng Culliton, B. J. Science 1978,201,235-39. Elmore, D.; Phillips, F. M. Science 1987, 236,54340. Taylor, R. E. Anal. Chern. 1987,59, 317A331A. Waldrop, M. M. Science 1988,241,1750-51.
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